1. Field of the Invention
The present invention relates to a method of manufacturing polyphenylene ethers by an oxidative coupling reaction of monomeric phenols.
2. Discussion of the Background
Polyphenylene ethers (PPEs) have become very important as high-temperature-resistant thermoplastics. Numerous methods for preparing them are known. See for example, Buehler, B., "Spezialplaste", Akademie-Verlag, Berlin (1978). The prevalent industrial methods are those in which diortho-substituted phenols are oxidatively coupled using copper-amine complexes in the presence of oxygen.
The method claimed in German OS No. 15 70 683 involves an oxygen transfer agent which is a catalyst comprised of a basic amine copper (II) salt complex of the formula EQU Cu(OH)X . RR'NH
where X is an acid anion, and R and R' are alkyl groups, or R' may be hydrogen. The active complex is prepared starting with copper (II) salts by mixing the a salt with copper (II) hydroxide in the presence of the primary or secondary amine; or by adding a base to the copper (II) salt in the presence of the amine. It is also possible for the same complex to be formed during the polycondensation, if one starts with a copper (I) salt and an amine of formula RR'NH and then oxidizes the resulting complex. Complexes of the indicated amines with non-basic copper (II) salts or copper (II) hydroxide are not active. Practically any primary and secondary mono- and polyamines are suitable, with the exception of amines which have an aryl substituent directly bound to the amine nitrogen. Thus, suitable types of amines include mono- and dialkylamines, mono- and dialkenylamines, mono- and dibenzylamines, mono- and diethanolamine, morpholine, multiply N-substituted alkylenediamines, diethylenetriamine, cyclohexylenediamine, and, in some cases, N-alkylated aminoalkylpyridines. The reaction is ordinarily terminated by adding a mineral acid or a base. German Pat. No. 15 70 683 disclosed the first industrially feasible method of preparing PPEs.
When this method was employed on a large scale, however, various disadvantages were soon apparent. These led to development of more active copper catalyst complexes. Typically, such complexes are comprised of the following (See German OS No. 25 05 328, corresponding to U.S. Pat. No. 4,028,341):
(a) a copper salt, usually copper bromide; PA1 (b) an alkali bromide or alkaline earth bromide (particularly if there is no bromide in the copper salt); PA1 (c) a secondary diamine, such as, e.g., N,N'-di-tert-butylethylenediamine; PA1 (d) a tertiary monoamine, e.g., N-butyldimethylamine; and, optionally, PA1 (e) additional amines, phase transfer agents, etc. PA1 oxidatively coupling phenols of the formula ##STR1## wherein R.sub.3 and R.sub.4 are H or CH.sub.3 ; and R.sub.1 and R.sub.2 are either n- or iso-alkyl groups with 1-6 carbon atoms or R.sub.1 is hydrogen and R.sub.2 is a tertiary alkyl group with 4-6 carbon atoms, in the presence of oxygen or oxygen-containing gas mixtures, in an organic solvent in the presence of a copper-amine catalyst comprising PA1 (a) anhydrous or hydrated CuCl or CuCl.sub.2 ; PA1 (b) a secondary diamine of the formula EQU R.sub.b --NH--R.sub.a --NH--R.sub.c PA1 wherein R.sub.a is a C.sub.2 -C.sub.4 alkylene group, and R.sub.b and R.sub.c are secondary or tertiary alkyl groups with 1-10 carbon atoms; and PA1 (c) morpholine; and wherein said organic solvent comprises a mixture of a C.sub.6 -C.sub.8 aromatic hydrocarbon and a C.sub.1 -C.sub.4 aliphatic alcohol in a volumetric ratio of 92.5:7.5 to 50:50.
It is well known to those skilled in the art and can be considered proven that copper bromides are more active than copper chlorides, and that the activity of copper chlorides can be increased by addition of materials which can liberate bromide ions (see German Pat. No. 2,228,071).
The most active copper bromide complexes contain a secondary diamine and an open-chain aliphatic tertiary amine, e.g. triethylamine or N-substituted butylamines. In German Pat. No. 2,505,328, it is expressly stated that the presence of bromide ions is important. The lower activity of copper chloride is also noted. Accordingly, CuBr.sub.2 is claimed in German Pat. No. 2,505,328 to be the preferred copper salt prepared, for example, by reaction of basic copper carbonate with hydrobromic acid. However, the use of hydrobromic acid, bromine, or even CuBr.sub.2 is associated with major corrosion problems.
German OS No. 27 38 889 discloses an improvement to the "bromide" activation catalyst system described in German Pat. No. 2,505,328. The improvement consists of additionally adding a secondary monoamine. However, the improved method retains the disadvantage of corrosion problems, and has the further disadvantage of requiring that the amine mixture be processed following the oxidative coupling.
The general teaching of activating a catalyst complex comprised of a copper salt, a secondary diamine, a tertiary monoamine, and optionally a secondary monoamine, by means of bromide ions is also seen in European Pat. No. 0 103 154.
There are also methods devised by the present Applicant wherein bromide ions enhance the activity of the catalyst. In these methods, the bromide ion is introduced in the form of the hydrobromide of morpholine, and morpholine is recommended as an amine ingredient of the copper complex (see German OS Nos. 32 24 692 and OS 32 24 691).
Finally, German Pat. No. 33 14 711 describes the oxidative coupling of monobasic phenols in aromatic solvents with the aid of a catalyst complex prepared from a copper salt and a secondary diamine. The use of tertiary and/or secondary monoamines is disadvantageous, according to the teaching of this patent.
The use of alkali bromides or alkaline earth bromides to activate the catalyst complex leads to complications in subsequent processing, because the presence of bromide ions in the end product is very detrimental to product quality.
It is known from U.S. Pat. No. 4,385,167 that difficulties are presented when catalysts based on copper salts and alkali bromides are used. To alleviate these difficulties, it is proposed to add sulfites or bisulfites to the catalyst.
European OS No. 0 150 022 discloses a pure crystalline complex prepared from monovalent copper, an N,N'-disubstituted ethylenediamine, and a phenolate group. This complex can be prepared only in the absence of moisture and oxygen, and is also suitable for the oxidative coupling reaction di-ortho-substituted phenols.
A need exists, therefore, for a catalyst which does not have the corrosion problems associated with bromide ions and yet is a economical catalyst.
When pure oxygen is used as the oxidant, there is a risk of explosion, in addition to the added cost in comparison to air. However, a number of state-of-the-art methods require pure oxygen, whereas in others the reaction rate drops drastically when one switches to air, and the PPE preparation becomes uneconomical.
A need exists therefore for a method of preparing PPEs which does not use pure oxygen.